A conventional transmission for a tractor is disclosed in U.S. Pat. No. 4,513,834 to Hayashi et al. Provided at the side surface of this transmission is a hydro-static transmission (HST) system. This system includes an output shaft, which is inserted into the transmission case, to drive the differential gears of the transmission and subsequently the axles of the tractor.
This transmission however, suffers from many disadvantages which are overcome by the present invention. First, this construction divides the transmission case vertically into two portions; one which houses a speed reduction gear train, and the other houses a differential gear. This construction also includes bearing bores which are made completely circular during a molding process. Hence, it is impossible to use a trimming die for casting the transmission and therefore, mass-produce the same by use of a metallic mold. Additionally, during assembly of the transmission, the worker is required to insert his hands into the transmission case for placement of the gears and their associated shafts.